Self-excited mixer circuit using field effect transistor

ABSTRACT

A self-excited mixer circuit using a dual gate type field effect transistor, in which an inductive impedance element inductive with respect to a local oscillation frequency, a first capacitive element and a second capacitive element are connected across the drain and the second gate, across the drain and the source, and across the second gate and the source, respectively, of field effect transistor to constitute an oscillation circuit across the drain and the second gate of transistor, so as to derive an intermediate frequency signal from the drain of the transistor in response to the application of the radio frequency signal to the first gate of transistor.

This invention relates to self-excited mixer circuits used inhigh-frequency systems of FM receivers, television receiving sets andthe like.

As is commonly acknowledged, a self-excited mixer circuit is lessexpensive than a separately excited mixer circuit. This is because, inthe case of the former, the function of local oscillation and thefunction of frequency conversion are done by a single active element,whereas, in the case of the latter, two active elements are required forachieving these two respective functions. However, successful productionof such self-excited mixer circuit has been encountered withconsiderable technical difficulty due to the fact that the optimum biaspoint for the single active element for determining the rate of feedbackof a local oscillation signal mixed with a radio frequency signaldiffers from that for determining the stability of sustainedoscillation, and differs also from that for determining the conversiongain. Because of the above technical difficulty, a self-excited mixercircuit using a field effect transistor (referred to hereinafter as anFET) has not been developed yet, although that using a bipolartransistor has been developed already and is presently put intopractical use.

However, such a conventional self-excited mixer circuit using a bipolartransistor has not been satisfactory in the function of eliminatingcross modulation and had to be improved in this function. It hastherefore been strongly demanded to realize a self-excited mixer circuitusing an FET which is better in its cross modulation eliminating abilitythan that using a bipolar transistor, in principle.

Incorporation of the conventional self-excited mixer circuit using thebipolar transistor in a tuner such as a television tuner or a radiotuner has resulted in insufficient tuner performance against crossmodulation interference, internal modulation interference and otherundesirable interference due to the non-linear characteristic of thebipolar transistor used as the active element.

It is therefore an object of the present invention to provide aself-excited mixer circuit using an FET which is better in its crossmodulation eliminating ability than that using a bipolar transistor.

Another object of the present invention is to provide a self-excitedmixer circuit which obviates the aforementioned prior art defect andimproves the tuner performance against various interference includingcross modulation interference and internal modulation interference.

With these objects in view, there is proposed according to one aspect ofthe invention a stably operable self-excited mixer circuit of highconversion gain in which a dual gate type FET is used as its activeelement and a characteristic oscillation circuit is connected across thesecond gate and the drain of the FET so as to suppress abnormaloscillation at an intermediate frequency and permit the supply of alocal oscillation signal in a most suitable amount.

According to another aspect of the invention, there is proposed aself-excited mixer circuit suitable for incorporation in thehigh-frequency system of an FM receiver, a television receiving set orthe like, in which a dual gate type FET is used as its active element soas to provide an improved cross modulation eliminating ability, and alocal oscillation frequency signal feedback loop is isolated from aradio frequency signal input section thereby obviating mutualinterference therebetween.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention made by referring to the preferredembodiments when taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 shows a basic circuit of a first embodiment of the self-excitedmixer circuit according to the present invention using a dual gate typeFET;

FIG. 2 is a practical circuit diagram of the first embodiment of thepresent invention;

FIG. 3 is a circuit diagram of a second embodiment of the presentinvention;

FIG. 4 shows a circuit diagram of a third embodiment of the presentinvention;

FIG. 5 shows a circuit diagram of a fourth embodiment of the presentinvention;

FIG. 6 is a circuit diagram of a fifth embodiment of the presentinvention; and

FIG. 7 is a circuit diagram of a sixth embodiment of the presentinvention.

Referring first to FIG. 1 illustrating a first embodiment of the presentinvention, there is shown a self-excited mixer circuit using a dual gatetype FET. In FIG. 1, the symbol FET designates a dual gate type FEThaving a first gate G1 and a second gate G2, and the symbols D and Sdesignate a drain and a grounded source respectively.

FIG. 1 shows a basic circuit form of part of a fourth embodiment of theself-excited mixer circuit according to the present invention. In FIG.1, the mixer circuit is shown with its D.C. circuit part, that is, thebias circuits for the first and second gates G1 and G2, and the drain D,omitted, to illustrate the basic arrangement of its high-frequencycircuit part.

Referring to FIG. 1, a dual gate type FET is grounded at its drain Dthrough a capacitor C5 and also through a π-type IF tuning sectionincluding an inductor L50 and a capacitor C50. An output signal ofintermediate frequency appears at an IF output terminal TIF connected tothe connection point of the inductor L50 and capacitor C50. The drain Dis connected to the second gate G2 of FET through a coupling capacitorC10 and an inductive impedance element LE2, and the connection point ofthe capacitor C10 and inductive impedance element LE2 is groundedthrough a capacitive impedance element CE10. The dual gate type FET isgrounded at its second gate G2 through another capacitive impedanceelement CE20.

In operation, a high-frequency voltage appearing at the drain D of FETis divided by the capacitor C10 and capacitive impedance element CE10 tobe fed back to the second gate G2 of FET through the inductive impedanceelement LE2 and capacitive impedance element CE20 while oscillating at aresonance frequency which is determined by the values of the capacitorC10, capacitive impedance elements CE10 and CE20, inductive impedanceelement LE2, capacitor C5, drain impedance of FET, and second gateimpedance of FET. A radio frequency signal transmitted to an RF inputterminal TRF is mixed with the local oscillation frequency signal in theFET, and after being amplified by the FET, the resultant outputappearing at the drain D of FET passes through the π-type IF tuningsection consisting of the capacitor C5, drain impedance, inductor L50and capacitor C50 to appear as an intermediate frequency signal at theIF output terminal TIF.

The capacitor C10 functions to ensure satisfactory impedance separationbetween the intermediate frequency band and the total oscillationfrequency band and has a relatively small capacitance value when thereis a small frequency difference between the intermediate frequency bandand the local oscillation frequency band.

The capacitive impedance element CE20 connected to the second gate G2 ofFET may be unnecessary when this second gate G2 has a capacitiveimpedance. However, the capacitive impedance element CE20 is preferablyconnected to the second gate G2 of FET with a view to deal with such anindefinite factor as fluctuation of the property of the FET.

In an application to a tuner such as a television tuner, it is necessaryto make the local oscillation frequency variable depending on thetelevision channels. In sucha case, the inductance value of theinductive impedance element LE2 or the capacitance value of thecapacitive impedance element CE10 may be made variable.

FIG. 2 shows a practical circuit structure of the first embodiment shownin FIG. 1, in which a capacitor CE21 is used as the capacitive impedanceelement CE20, and a variable capacitance diode VCD is used as thecapacitive impedance element CE10 so as to make variable the localoscillation frequency. In FIG. 2, the same symbols are used to designatethe same or like parts appearing in FIG. 1.

The D.C. circuit part of the mixer circuit includes a power supplyterminal B from which bias voltage is supplied to the first gate G1 ofthe FET through bias resistors R2 and R3, and to the ground gate G2 ofFET through bias resistors R1 and R4. Voltage is supplied from the lowersupply terminal B to the drain D of FET through a choke coil CH1 and IFtuning coil L50. Tuning voltage is applied from a tuning voltage inputterminal T1 to the variable capacitance diode VCD through a resistor R6to vary the capacitance of this diode VCD. Bypass capacitors C6, C7 andC8 are connected across the power supply terminal B and ground, acrossthe source S of FET and ground, and across the tuning voltage inputterminal T1 and ground, respectively.

In the high-frequency circuit part of the mixer circuit, the inductiveimpedance element LE2 and capacitive impedance element CE20 shown inFIG. 1 are replaced by a resonant inductor LE21 and a capacitor CE21respectively. Further, the capacitive impedance element CE10 shown inFIG. 1 is replaced by a group of variable capacitance elements,including a capacitor CE11 and variable capacitance diode VCD, which actto block the flow of direct current and control the variable localoscillation frequency range. In FIG. 2, the local oscillation frequencyis varied by the voltage applied to the variable capacitance diode VCD,so that one of intermediate frequency signals of selected frequency canbe derived from the IF output terminal TIF in response to theapplication of one of a plurality of radio frequency signals to the RFinput terminal TRF.

FIG. 3 shows a second embodiment of the present invention which is apartial modification of the first embodiment shown in FIGS. 1 and 2.Referring to FIG. 3, the inductive impedance element LE2 shown in FIG. 1is replaced by a parallel resonance circuit including a group ofvariable capacitance elements. This resonant circuit comprises aresonant inductor LE22 connected in parallel with the capacitor CE11 andvariable capacitance diode VCD. It will be noted that the positions ofthe capacitor CE11 and variable capacitance diode VCD differ from thoseshown in FIG. 2. In FIG. 3, capacitors CE12 and CE21 are respectivelyused as the capacitive impedance elements CE10 and CE20 shown in FIG. 1.

In the mixer circuit shown in FIG. 3, the capacitance value of thevariable capacitance diode VCD connected in parallel with the resonantinductor LE22 is varied to vary the equivalent inductive value of theparallel resonance circuit thereby varying the local oscillationfrequency. An inductor L3 is provided to ground the variable capacitancediode VCD in D.C. fashion.

FIG. 4 shows a third embodiment of the present invention which is also apartial modification of the first embodiment shown in FIGS. 1 and 2.Referring to FIG. 4, the inductive impedance element LE2 shown in FIG. 1is replaced by a series resonance circuit including a resonant inductorLE22 connected in series with the capacitor CE11 and variablecapacitance diode VCD. It will be noted that the positions of thecapacitor CE11 and variable capacitance diode VCD differ from thoseshown in FIG. 2.

In the mixer circuit shown in FIG. 4, the value of voltage applied fromthe terminal T1 to the variable capacitance diode VCD through theresistor R6 is varied to vary the capacitance value of the variablecapacitance diode VCD thereby varying the equivalent inductance value ofthe series resonance circuit for varying the local oscillationfrequency. In FIG. 4, capacitors CE12 and CE21 are used respectively asthe capacitive impedance elements CE10 and CE20 shown in FIG. 1.

In FIG. 5 showing a fourth embodiment of the present invention, the samesymbols are used to designate the same or like parts appearing in FIGS.1 to 4.

The D.C. circuit part of the mixer circuit includes a D.C. power sourceVG2 for D.C. biasing the second gate G2 of the FET, another D.C. powersource VG1 for D.C. biasing the first gate G1 of FET, and another D.C.power source VD for controlling the drain D of FET. The power source VG2applied its voltage to the second gate G2 of FET through a resistor R1,and the power source VG1 applies its voltage to the first gate G1 of theFET through another resistor R2, while the power source VD applies itsvoltage to the drain D of FET through a choke coil L1 and a coil L2.

In the high-frequency circuit part of the mixer circuit, a radiofrequency signal is applied from an RF input terminal TRF to the firstgate G1 of FET through a coupling capacitor C1. A capacitive impedanceelement CE1, which is capacitive with respect to a local oscillationfrequency, is connected across the drain D of the FET and ground. Aninductive impedance element LE1, which is inductive with respect to thelocal oscillation frequency, is connected across the drain D and thesecond gate G2 of FET. Another capacitive circuit element CE2, which iscapacitive with respect to the local oscillation frequency, is connectedacross the second gate G2 of FET and ground. These circuit elements CE1,LE1 and CE2 constitute a feedback loop which determines the localoscillation frequency. An intermediate frequency (IF) tuning sectionconsisting of a coil L2 and a capacitor C2 is connected to the drain Dof the FET, and an IF output terminal TIF is connected through acoupling capacitor C3 to the connection point of the coil L2 andcapacitor C2. An intermediate frequency component appears at this IFoutput terminal TIF.

In operation, a radio frequency signal transmitted to the RF inputterminal TRF is mixed with a local oscillation frequency signal producedby the feedback loop consisting of the circuit elements CE1, LE1 and CE2connected across the drain D and the second gate G2 of FET, and theresultant signal of intermediate frequency representing the frequencydifference between the radio frequency signal and the local oscillationfrequency signal is trapped by the IF tuning section consisting of thecircuit elements L2 and C2 connected to the drain D of FET to appear atthe IF output terminal TIF.

The mixer circuit according to the present invention is thusspecifically featured by the fact that the dual gate type FET capable ofsatisfactorily eliminating cross modulation is used as its activeelement, and the local oscillation frequency signal feedback loop isisolated from the radio frequency signal input section. By virtue of theuse of the dual gate type FET, the self-excited mixer circuit of thepresent invention exhibits a cross modulation eliminating functionbetter than that of the aforementioned conventional one using thebipolar transistor. Also, by virtue of the isolation of the localoscillation frequency signal feedback loop from the radio frequencysignal input section by interposing all the feedback circuit elementsbeteeen the drain and the second gate of FET, input matching isfacilitated, and undesirable interference between the RF input sectionand the local oscillation section can be avoided to improve the mixingcharacteristic.

In an application of the mixer circuit of the present invention to, forexample, a television tuner, it is necessary to make the localoscillation frequency variable so as to meet a variety of televisionchannels. This is easily done by making variable the inductance value ofthe inductive impedance element LE1 or the capacitance value of thecapacitive impedance element CE2 depending on the television channels.

FIG. 6 shows a fifth embodiment of the self-excited mixer circuitaccording to the present invention, and the same symbols are used inFIG. 6 to designate the same or like circuit elements appearing in FIG.5, since this embodiment is a modification of the fourth embodiment.This fifth embodiment is the same as the fourth embodiment in its D.C.circuit part, but differs from the fourth embodiment in itshigh-frequency circuit part as described below.

Referring to FIG. 6, a coupling capacitor C10 is connected at oneterminal thereof to the drain D of dual gate type FET. Connected acrossthe other terminal of this coupling capacitor C10 and the second gate G2of FET is a resonance circuit which determines the local oscillationfrequency of the mixer circuit. This resonance circuit is composed of acapacitor C20 and a coil L20 which are connected in parallel andcorrespond to the inductive impedance element LE1 employed in the firstembodiment. A DC-component blocking capacitor C4 is connected across theother terminal of the coupling capacitor C10 and the ground. Connectedalso across the other terminal of the coupling capacitor C10 and theground is another resonance circuit composed of a coil L30 and acapacitor C30 which are connected in series to resonate in the vicinityof the intermediate frequency. The aforementioned circuit elementsconnected across the drain D and the second gate G2 of FET provide thefeedback loop.

In operation, a radio frequency signal transmitted to the RF inputterminal TRF is mixed with the local oscillation frequency signalproduced by the feedback loop consisting of the circuit elements C10,C20 and L20 connected across the drain D and the second gate G2 of FET,and the resultant signal of intermediate frequency representing thefrequency difference between the radio frequency signal and the localoscillation frequency signal is trapped by the IF tuning sectionconsisting of the circuit elements C2 and L2 to appear at the outputterminal TIF.

The fifth embodiment of the present invention is thus specificallyfeatured by the fact that the circuit elements C10, C20 and L20constituting the feedback loop are connected across the drain D and thesecond gate G2 of FET. The circuit constant of the resonance circuitconsisting of the capacitors C4, C30 and coil L30 is preferablydetermined so that this resonance circuit can be rendered resonant atsubstantially the intermediate frequency. When the circuit constant ofthe resonance circuit is so determined, this resonance circuit providessuch an extremely high impedance against the local oscillationfrequency, which is equivalent to the presence of an impedance in thecapacitor C4. Therefore, this resonance circuit traps the intermediatefrequency so that the signal of intermediate frequency can be preventedfrom entering the feedback loop, and the efficiency of frequencyconversion can be improved.

According to this fifth embodiment of the present invention, the localoscillation frequency component can be most suitably divided by thecapacitors C4 and C10 to be fed back to the second gate G2 of FETwithout in any way affecting the intermediate frequency component. Thus,the local oscillation frequency component can be fed back with highstability thereby ensuring a high conversion gain. An experiment wasconducted to prove the advantage of the fifth embodiment of the presentinvention over the conventional one. The experimental results provedthat the gain of frequency conversion was 16 to 18 dB, and the frequencyband width BW was 6 to 8 MHz in the hiband of the VHF range. Theparallel resonance circuit consisting of the capacitor C20 and coil L20in FIG. 6 may be replaced by a series resonance circuit which exhibitsalso an entirely similar effect. As described with reference to thefourth embodiment, the fifth embodiment is also applicable to atelevision tuner. In such a case, the value of the capacitor C20 and/orcoil L20 must be made variable.

It will be understood from the above description that the fifthembodiment of the present invention ensures also a high conversion gaindue to the fact that the local oscillation frequency component can befed back in a most suitable amount without lowering the impedance at theintermediate frequency of the feedback loop connected across the drainand the second gate of FET.

FIG. 7 shows a sixth embodiment of the self-excited mixer circuitaccording to the present invention, and the same symbols are used inFIG. 7 to designate the same or like parts appearing in FIG. 6, sincethis embodiment is a modification of the fifth embodiment. This sixthembodiment is the same as the fifth embodiment in its D.C. circuit part,but differs slightly from the fifth embodiment in its high-frequencycircuit part as described below.

Referring to FIG. 7, the feedback loop connected across the drain D andthe second gate G2 of dual gate type FET includes, in addition to thecoupling capacitor C10 and parallel resonance circuit consisting of thecapacitor C20 and coil L20, another parallel resonance circuitconsisting of a capacitor C40 and a coil L40 for preventing feedback ofthe intermediate frequency component. This second parallel resonancecircuit is connected in series with the first parallel resonance circuitwhich becomes resonant at the local oscillation frequency. Although thesecond resonance circuit is shown disposed nearer to the second gate G2of FET than the first resonance circuit, the operation and effect arethe same even when the above relation is reversed.

In operation, a radio frequency signal transmitted to the RF inputterminal TRF is similarly mixed with the local oscillation frequencysignal produced by the feedback loop, and the resultant signal ofintermediate frequency appears similarly at the IF output terminal TIF.In this sixth embodiment, the circuit constant of the second resonancecircuit consisting of the capacitor C40 and coil L40 is selected so thatthe circuit becomes resonant at substantially the intermediatefrequency, and thus, any appreciable signal component of intermediatefrequency would not be fed back through the feedback loop when lookedfrom the drain D of FET. It is therefore possible to prevent parasiticoscillation at the intermediate frequency and increase the conversiongain. The local oscillation frequency component is divided by thecapacitors C10 and C4 so that the local oscillation frequency componentcan be fed back in a most suitable amount without lowering the impedanceagainst the intermediate frequency. Thus, an intermediate frequencycomponent which may appear in the feedback loop can be trapped by boththe first resonance circuit consisting of L30 and C30 and the secondresonance circuit consisting of L40 and C40 disposed in the feedbackloop, which contributes to the desired improvement in the efficiency offrequency conversion like the aforementioned fourth and fifthembodiments. The valve of the capacitor C20 and/or coil L20 may be madevariable.

It will be understood from the above description of the sixth embodimentof the present invention that undesirable parasitic oscillation at theintermediate frequency can be prevented due to the fact that thefeedback loop connected across the drain D and the second gate G2 of FETexhibits a high impedance at the intermediate frequency. Further, a highconversion gain can be obtained due to the fact that the localoscillation frequency component can be fed back in a most suitableamount without lowering the impedance at the intermediate frequency ofthe feedback loop connected across the drain D and the second gate G2 ofFET. The second parallel resonance circuit consisting of the capacitorC40 and coil L40 in FIG. 7 may be replaced by a series resonance circuitwhich exhibits also an entirely similar effect.

It will be understood from the foregoing detailed description that theself-excited mixer circuit according to the present invention isadvantageously applicable to a television tuner or radio tuner forimproving the tuner performance against cross modulation interferenceand internal modulation interference resulting from the non-linearity ofthird and higher orders. Especially, in a VHF tuner, the picture carrierfrequency, sound carrier frequency and local oscillation frequencyallotted to the channel No. 6 of the television channel system employedin the United States of America tend to interfere to produce a colorbeat interference due to the above non-linearity, and similarly, thesound carrier frequency and local oscillation frequency allotted to thechannel No. 4 of the television channel system employed in the EuropeanCommunities tend to interfere to produce a color beat interference dueto the above non-linearity. The mixer circuit according to the presentinvention is especially effective in obviating such a color beatinterference.

What is claimed is:
 1. A self-excited mixer circuit comprising a dualgate field effect transistor having a source, a drain, a first gate anda second gate, said field effect transistor being grounded at saidsource, a capacitor connected at one terminal thereof to the drain ofsaid field effect transistor, a first capacitive impedance elementconnected across the other terminal of said capacitor and ground, aninductive impedance element connected across the other terminal of saidcapacitor and the second gate of said field effect transistor, saidcapacitive impedance element and said inductive impedance elementconstituting an oscillation circuit across the drain and the second gateof said field effect transistor, means for applying a radio frequencysignal to the first gate of said field effect transistor, and means forderiving the resultant signal of a predetermined intermediate frequencyfrom the drain of said field effect transistor, wherein circuitconstants of said impedance elements are provided so as to be able toderive said predetermined intermediate frequency from the drain uponreceiving said radio frequency signal at the first gate of said fieldeffect transistor.
 2. A mixer circuit as claimed in claim 1, furthercomprising a second capacitive impedance element connected across thesecond gate of said field effect transistor and ground to form aColpitts oscillation circuit.
 3. A mixer circuit as claimed in claim 1,wherein said first capacitive impedance element is a variablecapacitance element.
 4. A mixer circuit as claimed in claim 1, whereinsaid inductive impedance element is a variable inductance element.
 5. Amixer circuit as claimed in claim 1, wherein said inductive impedanceelement comprises a group of elements including a variable capacitanceelement connected in parallel with an inductor.
 6. A mixer circuit asclaimed in claim 1, wherein said inductive impedance element comprises agroup of elements including a variable capacitance element connected inseries with an inductor.
 7. A mixer circuit as claimed in claim 1,wherein a series circuit composed of a second capacitive impedanceelement and a second inductive impedance element for being resonated atthe predetermined intermediate frequency is connected to the drain ofsaid field effect transistor, and predetermined intermediate frequencyis derived from a connection point between said second capacitiveimpedance and said second inductive impedance elements.
 8. Aself-excited mixer circuit comprising a dual gate type field effecttransistor having a source, a drain, a first gate and a second gate,said field effect transistor being grounded at said source, an inductiveimpedance element connected across the drain and the second gate of saidfield effect transistor, said inductive impedance element beinginductive with respect to a local oscillation frequency, a firstcapacitive impedance element connected across the drain of said fieldeffect transistor and ground, a second capacitive impedance elementconnected across the second gate of said field effect transistor andground, said inductive impedance element and said first and secondcapacitive impedance elements constituting an oscillation circuit acrossthe drain and the second gate of said field effect transistor, means forapplying a radio frequency signal to the first gate of said field effecttransistor, and means for deriving the resultant signal of apredetermined intermediate frequency from the drain of said field effecttransistor, wherein the values of said inductive impedance element andsaid first and second capacitive impedance elements are provided so asto be able to derive said predetermined intermediate frequency from thedrain upon receiving said radio frequency signal at the first gate ofsaid field effect transistor.
 9. A mixer circuit as claimed in claim 8,wherein the inductance value of said inductive impedance element isvariable.
 10. A mixer circuit as claimed in claim 8, wherein thecapacitance value of said first capacitive impedance element isvariable.
 11. A mixer circuit as claimed in claim 8, wherein a seriescircuit composed of a second capacitive impedance element and a secondinductive impedance element for being resonated at the predeterminedintermediate frequency is connected to the drain of said field effecttransistor, and the predetermined intermediate frequency is derived froma connection point between said impedance and inductive impedanceelements.
 12. A self-excited mixer circuit comprising a dual gate typefield effect transistor having a source, a drain, a first gate and asecond gate, said field effect transistor being grounded at said source,a coupling capacitor connected at one terminal thereof to the drain ofsaid field effect transistor, a first resonance circuit connected acrossthe other terminal of said coupling capacitor and ground to be renderedresonant at substantially an intermediate frequency, a second resonancecircuit connected across said other terminal of said coupling capacitorand the second gate of said field effect transistor to determinesubstantially a local oscillation frequency, means for applying a radiofrequency signal to the first gate of said field effect transistor, andmeans for deriving the resultant signal of a predetermined intermediatefrequency from the drain of said field effect transistor, wherein thevalue of said coupling capacitor and the values of the elementscomprising said second resonance circuit are provided so as to be ableto derive said predetermined intermediate frequency from the drain uponreceiving said radio frequency signal at the first gate of said fieldeffect transistor.
 13. A mixer circuit as claimed in claim 12, whereinsaid second resonance circuit comprises an inductance element and acapacitance element connected in parallel.
 14. A mixer circuit asclaimed in claim 12, wherein said first resonance circuit comprises aninductance element and a capacitance element connected in series.
 15. Amixer circuit as claimed in claim 12, wherein said second resonancecircuit is a variable resonance circuit.
 16. A mixer circuit as claimedin claim 12, wherein a series circuit composed of a capacitive impedanceelement and an inductive impedance element for being resonated at thepredetermined intermediate frequency is connected to the drain of saidfield effect transistor, and the predetermined intermediate frequency isderived from a connection point between said impedance and inductiveimpedance elements.
 17. A self-excited mixer circuit comprising a dualgate field effect transistor having a source, a drain, a first gate anda second gate, said field effect transistor being grounded at saidsource, a coupling capacitor connected at one terminal thereof to thedrain of said field effect transistor, a first resonance circuitconnected across the other terminal of said coupling capacitor andground to be rendered resonant at substantially an intermediatefrequency, a parallel resonance circuit connected across said otherterminal of said coupling capacitor and the second gate of said fieldeffect transistor to determine substantially a local oscillationfrequency, a second resonance circuit connected in series with saidparallel resonance circuit across said other terminal of said couplingcapacitor and said second gate of said field effect transistor and to berendered resonant at substantially said intermediate frequency, meansfor applying a radio frequency signal to the first gate of said fieldeffect transistor, and means for deriving the resultant signal of apredetermined intermediate frequency from the drain of said field effecttransistor, wherein the value of said coupling capacitor and the valuesof the elements comprising said parallel resonance circuit are providedso as to be able to derive said predetermined intermediate frequencyfrom the drain upon receiving said radio frequency signal at the firstgate of said field effect transistor.
 18. A mixer circuit as claimed inclaim 17, wherein said first resonance circuit comprises an inductanceelement and a capacitance element connected in series.
 19. A mixercircuit as claimed in claim 17, wherein said parallel resonance circuitis a variable resonance circuit.
 20. A mixer circuit as claimed in claim17, wherein a series circuit composed of a capacitive impedance elementand an inductive impedance element for being resonated at thepredetermined intermediate frequency is connected to the drain of saidfield effect transistor, and the predetermined intermediate frequency isderived from a connection point between said impedance and inductiveimpedance elements.
 21. A mixer circuit as claimed in claim 17, whereinsaid second resonance circuit comprises a capacitive impedance and aninductive impedance connected in parallel.